The claim: Home Secretary Theresa May says: "Free movement makes it harder to control immigration, but it doesn't make it impossible to control immigration.

Reality Check verdict: Free movement does make it harder to control immigration. But leaving the EU would not necessarily mean abandoning free movement, and the government has not managed to meet its immigration ambitions even for non-EU migrants, over whom it has greater control.

Freedom of movement means people holding the passports of European Union member states may go and live, work or study anywhere else in the EU.

This compares with migrants from outside the EU, for whom the UK currently has a points-based system that allows in people with skills needed in the economy as well as students and some temporary migrants.

The UK is not signed up to the Schengen agreement, so passports are still checked at the borders.

EU passport-holders are not automatically allowed in, but the bar is set quite high if EU citizens are to be rejected - just having a criminal record is not enough, they must pose a current threat. Last year, 2,165 EU passengers were turned away.